1. Field of the Invention
This invention relates to an ion implantation system and method and more particularly to a system and method for optimizing the placement of dopant material upon a semiconductor surface area.
2. Background of the Relevant Art
Ion implantation is a semiconductor doping process whereby a plurality of dopant atoms are first ionized, then accelerated to velocities sufficient to penetrate the semiconductor surface and deposit within. Well known are semiconductors which can be altered in electrical behavior by the introduction of minute quantities of elemental materials called "dopants." Dopants generally come in either p-type or n-type. P-type dopants (including boron, Al, Ga, Tl, In and/or Si) produce what is commonly known as hole conductivity while n-type dopants (including phosphorous, arsenic and/or antimony) produce what is commonly known as electron conductivity. Combination of hole and electron-rich regions produce the desired devices such as transistors, resistors, diodes, capacitors, etc., which form the basis of semiconductor operation. Recent advances in semiconductor manufacture include fine-line geometries of the dopant materials placed on a substrate to form very large scale integrated devices.
Integrated circuits are generally formed by connecting numerous individual devices set forth by dopant implantation. A single wafer may contain several thousand devices which are diced and individually packaged as a single monolithic circuit. It is important that the doping process be accurately presented to the semiconductor area in order to ensure the monolithic circuit operates according to target design parameters. If doping does not bring about such operation, then the corresponding yields may be drastically reduced thereby adding to the cost of manufacture. Important factors relating to accurate doping are: (i) the need to control the number of doping ions introduced in the surface material, (ii) the need to control the uniformity of doping ions placed across the surface, (iii) the need to control the depth or concentration profile of dopant placed into the surface, and (iv) the need to ensure the doping pattern can be introduced as a maskable pattern on the surface. Each of these factors must be closely monitored and can be achieved by several different techniques. One technique may include bulk diffusion of dopant into non-masked surface regions. Another technique may involve direct sourcing of an ion beam at select fine-line areas into non-masked regions. The latter technique generally involves an ion beam implantation system.
An ion beam implantation system 10 is partially shown in FIG. 1. System 10 generally involves several subsystems. One subsystem is an ion source 12 which functions to supply an "ionized atom" of dopant material. Ion source 12 removes one or more electrons from the dopant atom thereby creating a positively charged ionized atom which can be extracted and accelerated to a target surface such as a semiconductor surface. Elements of an ion source include a relatively high pressure area 14 contained within a first chamber 16. Chamber 16 is configured to receive gaseous compound of selected dopant material. Depending upon the type of dopant used, either p-type of n-type, dopant can be introduced as a gas into chamber 16. The dopant-rich gas containing atoms of the desired species generally receives electrons generated from an ionizing filament 18. Filament 18 radiates electrons when it is heated to a specific temperature and when a specific extraction voltage is applied thereto. Collision, shown as numeral 32, between electrons and gas atoms results in the desired ionization within chamber 14. The ions are then extracted through an aperture 20 placed through the wall of an ionizing (or arc) area. A plurality of ions are extracted through use of an electrostatic field created by charged extraction electrodes 24 placed along ion path 26 as shown. So as to attract positively charged ions, extraction electrodes 24 must receive a negative potential with respect to the ion source. Thus, extraction electrodes 24 are preferably negatively charged relative to ion source 32.
The collision of electrons with dopant atoms may be caused by, e.g., an arc established between the walls of the chamber surrounding area 14 and filament 18. The electrons generally move in a spiral orbit to increase the collision rate with surrounding gas atoms. The mean free path is optimized by a magnetic field placed parallel to filament 18. Magnetic field occurs by powering magnets 30 via terminals 31 as shown.
FIG. 1 illustrates only one of many types of ion sources. Any type of ion source 12 may be used provided it contain the essential elements of having a high pressure area, a source of electrons to collide with desired atomic species within the area, an electric field to mobilize the electrons and atoms resulting in collision, and means for extracting ions from the source. The ion source illustrated herein may use a heated type filament 18 to generate the necessary arc within area 14. Thus, not only is filament 18 powered by a voltage supply connected to leads 32 to achieve emission, but also a powered heater 36 may be used to enhance emission.
Further shown in FIG. 1 is an analyzing magnetic 38. Magnet 38 may be placed adjacent to source 12 as shown in FIG. 1 or, alternatively, it can be placed subsequent an accelerated tube 40 (shown in FIG. 2 according to the present invention). In the latter arrangement, accelerator tube 40 is placed between magnet 38 and source 12. Either arrangement will prove suitable provided magnet 38 is adjusted in size and weight to accommodate delivered extractor energies. For example, if magnet 38 is placed between accelerator tube 40 and source 12, magnet 38 receives ion energies typically of lesser values (i.e., approximately 20 KV) than if magnet is placed downstream of accelerator tube 40 (wherein energies may be 200 KV or more).
Magnet 38 typically functions as a mass analyzer. Ion source often produces many different ions. Only one ion mass is to be selected and placed upon the target. Since ions at source 12 may differ by weight, they can be separated according to their mass. The electric force on the ions is a function of both the mass and charge of the ions such that ions are separated out from each other according to different mass-to-charge ratios. If, for example, a perpendicular magnetic field is utilized as shown in FIG. 1 to achieve mass selection, ions may be bent according to their mass and charge with the lighter ions 42 being bent more and the heavier ions 44 being bent less. An aperture 46 is placed at the output side of magnet 38, and the magnetic field is adjusted via power terminals 48 such that the desired ion goes through aperture 46 and undesirable ions are stopped.
Ion beam 26 is emitted from magnet 38 for possible subsequent processing. Namely, beam 26 may become accelerated upon a target area such as a semiconductor surface. Acceleration may be needed in order to ensure penetration of ions into the surface. The amount of acceleration is directly proportional to a beam current. A problem with many conventional high beam implantation systems is that the associated beam current is confined to be somewhat low. If beam current of beam 26 exceeds a specific amount, the resulting semiconductor surface may receive a large amount of positivewly charged ions during implant thereby resulting in certain defects such as, but not limited to, gate oxide degradation and breakdown. If the beam is scanned quickly across the semiconductor surface, conventional systems still may place non-uniform dopant density across some areas of the surface.
As shown in FIG. 3, electric field between the gate region and the substrate region (i.e., across the gate oxide) demonstrates a fairly linear relationship between beam current and electric field. As beam current increases, the amount of dopant ions placed within the polysilicon gate may also be proportionally increased. However, once beam current exceeds a certain value, then the resulting electric field will exceed a critical level, denoted as E.sub.max. Electric field exceeding E.sub.max is deemed to cause gate oxide breakdown whenever an applied gate voltage exceeds an unacceptably low amount. Susceptability to breakdown results from a large build up of negatively charged electrons attracted to the gate oxide/substrate interface configured below the gate region. The electrons are attracted by the large ion dopant level on the overlying gate, forcing the electrons to become trapped within the substrate region causing current to flow through the gate oxide from the gate-applied power source whenever the applied gate potential rises above a certain breakdown voltage amount. Build up of ion concentration (or "dopant level") exceeding the critical amount, E.sub.max, will allow undesirably low applied voltage to cause gate oxide breakdown. Gate voltages which are within the designer's range but which exceed the low breakdown voltage will cause the circuit to malfunction when it should not have done so. As shown in FIG. 3, most conventional ion beam implantation systems utilize beam currents that are less than five millamperes, the upper range being restricted by E.sub.max.
It is important in the manufacture of a semiconductor circuit that breakdown voltages be quite large in order to prevent malfunction of the circuit. A lower beam current can achieve this objective. However, higher beam currents allow faster doping across the semiconductor surface, thereby increasing throughput of the implantation process. High beam current implanters generally employ mechanical scanner which move the target relative to the implant beam. However, the mechanical scanners are usually too slow and cannot avoid the instantaneous charging or beam influx upon the target area. For example, the spin and scan speeds of Applied Materials, Inc. model no. AMT-9200 implanter are 1300 r.p.m. and 15 cm/sec, respectively. Unless spin exceeds nearly 5000 r.p.m., instantaneous current density influx can become significant. However, high spin rates requires time necessary to ramp to and from the higher rates. Further, a heavy spin wheel poses a mechanical force limitation beyond which the spin rate of the target cannot exceed without causing periodic damage to the spin mechanism. As such, there remains a need for high beam current implantation systems which do not prove detrimental in achieving an optimal circuit performance having higher breakdown voltage levels.